Climate change could make California wetter, study finds

California could get wetter — not drier — in coming decades, according to a UC Riverside study that upends some conventions about climate change.

The study, by climatology professor Robert Allen and graduate student Rainer Luptowitz, was published Thursday in the scientific journal Nature Communications. It ran climate forecasts through a suite of models, and projected that California will get 12 percent more rain through the end of this century than it did in the last two decades of the 20th century.

That conclusion runs counter to most previous climate forecasts, which show the state growing increasingly hot and arid. Instead, the UCR study states, California could experience what amounts to ongoing El Nino conditions, with warm, wet winters becoming the norm.

“My study kind of goes against prior studies,” Allen said. “It’s kind of the first study that suggests California might get wetter.”

Precipitation, however, probably won’t increase across the board, the authors noted. By their calculations, Northern California would get 14.1 percent more precipitation and Central California would get 15.2 percent more, but Southern California would see a slight decrease of 3.3 percent.

Nonetheless, all regions would get more water in the winter — in some cases much more. During the state’s typical rainy months of December, January and February, precipitation would climb by about a third in Northern and Central California would, while Southern California would see a surge of 11 percent, Allen said. Other months of the year could become drier.

Predicting California’s future climate is tricky, Allen said, because the state straddles the temperate zone, which is expected to become wetter in coming year, and the subtropics, which are likely to dry out. Hence the divide between the northern and southern parts of the state.

To study statewide precipitation, Allen and Luptowitz analyzed 38 climate models and selected about a dozen that most accurately simulate current California climatology. When they applied those to future climate conditions, their results were surprising. The models projected warming in the tropical eastern Pacific, which would shift the jet stream southward, steering storms toward California.

Allen said he plans to run newer climate models next year, to see if they produce the same findings. And he noted that there are questions his research can’t yet answer. The study analyzed only average precipitation, not rainfall patterns. So it doesn’t account for year-to-year variation such as long-term drought, or powerful atmospheric rivers.

Alexander Gershunov, a research meteorologist for Scripps Institution of Oceanography at UC San Diego, said that weather may get somewhat wetter, but his own research with fellow climate researcher Dan Cayan suggests that it will also be more severe, with dry periods punctuated by powerful storms.

“When you look at daily precipitation, we see two competing signals — a decrease in frequency of precipitation and increase in frequency of the most extreme events,” Gershunov said. “It means... more opportunity for drought and more opportunity for floods.”

The UCR study also didn’t analyze whether precipitation will come as rain or snow, either, Allen said, but rising temperatures tip the scale toward rain. That has profound implications for water use and storage, because the state mostly relies on winter snowpack to store and slowly release runoff.

“I did not distinguish liquid versus frozen precipitation, but these conclusions feature a lot of warming for the state,” he said. “Although there’s an increase in precipitation, it’s most likely going to be in the form of liquid rather than snow. And even if it falls as snow, it’s not likely to last as long as it currently does. We may need to rethink some of our water storage structures, our aqueducts, our reservoirs.”

And although the study projects more water for the Central Valley and other agricultural regions, it’s not all good news for farmers, he said. The benefits of extra rainfall could, literally, evaporate to increased heat, he warned.

“With a warming state, you would expect enhanced evaporation of soil moisture,” he said. “So even though there’s increase in precipitation based on my study, is that enough to offset the increased evaporative demand in a warmer world? And I don’t know the answer to that.”